Digital cameras and other image capture devices operate by capturing electromagnetic radiation and measuring the intensity of the radiation. The spectral content of electromagnetic radiation focused onto a focal plane depends on, among other things, the image to be captured, the illumination of the subject, the transmission characteristics of the propagation path between the image subject and the optical system, the materials used in the optical system, as well as the geometric shape and size of the optical system.
For consumer imaging systems (e.g., digital cameras) the spectral range of interest is the visible region of the electromagnetic spectrum. A common method for preventing difficulties caused by radiation outside of the visual range is to use ionically colored glass or a thin-film optical coating on glass to create an optical element that passes visible light (typically having wavelengths in the range of 380 nm to 780 nm). This element can be placed in front of the taking lens, within the lens system, or it can be incorporated into the imager package. The principal disadvantage to this approach is increased system cost and complexity.
A color filter array (CFA) is an array of filters deposited over a pixel sensor array so that each pixel sensor is substantially sensitive to only the electromagnetic radiation passed by the filter. A filter in the CFA can be a composite filter manufactured from multiple filters so that the transfer function of the resulting filter is the product of the transfer functions of the constituent filters. Each filter in the CFA passes electromagnetic radiation within a particular spectral range (e.g., wavelengths that are interpreted as red). For example, a CFA may be composed of red, green and blue filters so that the pixel sensors provide signals indicative of the visible color spectrum.
If there is not an infrared blocking element somewhere in the optical chain infrared (IR) radiation (typically considered to be light with a wavelength greater than 780 nm) may also be focused on the focal plane. Imaging sensors or devices based on silicon technology typically require the use of infrared blocking elements to prevent IR from entering the imaging array. Silicon-based devices will typically be sensitive to light with wavelengths up to 1200 nm. If the IR is permitted to enter the array, the device will respond to the IR and generate an image signal including the IR.
In current three-dimensional cameras, the depth information is captured separately from the color information. For example, a camera can capture red, green and blue (visible color) images at fixed time intervals. Pulses of IR light are transmitted between color image captures to obtain depth information. The photons from the infrared light pulse are collected between the capture of the visible colors.
The number of bits available to the analog-to-digital converter determines the depth increments that can be measured. By applying accurate timing to cut off imager integration, the infrared light can directly carry shape information. By controlling the integration operation after pulsing the IR light, the camera can determine what interval of distance will measure object depth and such a technique can provide the shape of the objects in the scene being captured. This depth generation process is expensive and heavily dependent on non-silicon, mainly optical and mechanical systems for accurate shutter and timing control.